Synchronizing circuit

ABSTRACT

A synchronizing circuit is disclosed which enables a desired phenomena to occur, such as the discharge of a flash illuminating means at a precise point along the path of travel of an article irrespective of the speed of the article in that path. 
     The circuit utilizes two spaced sensors upstream of the precise point. The sensors are operable to detect the passage of the article and each sensor is connected to respective counter. When sensor detects the passage of the article it starts its respective counter counting in one direction at one particular counting rate. When the second sensor detects the passage of the article it causes its respective counter to count in the opposite direction from the value of the count in the first count at a different but faster counting rate. The circuit includes gate means which determine when the count has returned to a predetermined count to then cause said phenomena to occur.

FIELD OF THE INVENTION

This invention relates to a synchronisation circuit and relatesparticularly but not exclusively to a synchronisation circuit used inthe art of photographing projectiles, such as bullets, so as toilluminate a projectile at a precise point in its path, regardless ofthe velocity of the projectile.

The circuit is generally applicable to cause any desired phenomena to beinitiated at a precise point in the path of travel of an article,regardless of the velocity of the article.

PRIOR ART

In the art of photographing projectiles such as bullets it has been verydifficult to ascertain exactly when the projectile reaches the field ofview of the camera. Therefore, it has sometimes been the practice toeither run the camera continuously if it is a movie camera and tocontinuously illuminate the area within the field of view of the cameraduring the whole of the time of travel of the projectile. This resultsin a waste of film and prolonged used of the illuminating lamps whichhave a rather short life expectancy. This prolonged use, in turn,requires that the lamps be replaced more frequently than if the areawithin the field of view of the camera is illuminated only when theprojectile is within that area. If the camera is a still camera, then itis important that the shutter of the camera be released exactly when theprojectile is within the field of view of the camera. The correct timingof these functions has been difficult. This is because the precisevelocity of an individual bullet is unknown before it is discharged. Ifthe precise velocity were known, then it would be a simple matter tocalculate the time take after discharge to reach a given point and hencethe camera and the lamps could be activated at that time. The problem isfurther complicated in that distinctly different velocity projectilesmay be required to be photographed one after the other and therefore atotally different time setting is required.

Accordingly it is an object of the invention to provide a synchronizingcircuit to cause a phenomena to occur at a precise point in the path oftravel of an article, irrespective of the speed of movement of thatarticle.

STATEMENT OF THE INVENTION

Therefore in accordance with a broad aspect of the present inventionthere is provided a synchronising circuit for causing a phenomena tooccur at a precise point in the path of travel of an article regardlessof the speed of movement of the article in that path, comprising firstand second spaced sensors upstream of said precise point and operable todetect the passage of an article therepast and to provide output signalsconsequent thereon, a counting circuit which provides a count in onedirection when the first sensor provides an output signal, and whichcounts in the opposite direction from the value of the count firstcounted, when the second sensor provides an output signal and circuitmeans responsive to the counting circuit returning to a predeterminedcount to initiate the occurrence of said phenomena.

Most preferably the article is a bullet and the phenomena isillumination of the bullet by a suitable flash unit. It is alsopreferred that the sensors be magnetic sensors.

BRIEF DESCRIPTION OF DRAWINGS

In order that the invention can be more clearly ascertained a preferredembodiment for use in high speed movie photography of a bullet will nowbe described with reference to the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a system used for taking movie film ofa bullet as it passes a particular location.

FIG. 2 is a diagrammatic view of part of a sensing head used for sensingthe passing of a bullet.

FIGS. 3a, 3b and 3c are a detailed circuit diagram of a preferredelectronic circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to FIG. 1 there is shown a bullet 1 travelling along atrajectory 2 towards a target 3 mounted within a catcher box 4. Thecatcher box 4 has a window or opening 5 therein at one side so that acine camera 6 can photograph the bullet passing through the catcher box4 and striking the target 3. The cine camera 6 includes a known flashilluminating means not shown. Two sensors S₁ and S₂ are placed upstreamfrom the catcher box 4 and the sensors S₁ and S₂ are placed apart aknown distance X. The sensors S₁ and S₂ are connected with an electroniccircuit used for synchronising the occurrence of the flash to illuminatethe projectile when it is in the field of view of the camera 6 withinthe catcher box 4. The synchronising circuit may also be used to startthe operation of the cine camera 6 if desired.

The sensors S₁ and S₂ are electromagnetic sensors which detect thedisturbance in the field which emanates from each, consequent on thepassage of the bullet therepast. The sensors S₁ and S₂ are hollowsensors and the trajectory 2 of the bullet 1 passes therethrough. Atypical sensor construction is shown in FIG. 2 where only sensor S₁ isshown. Sensors S₁ and S₂ are mounted on a common supporting member 10which is a plastics material tube, 700 millimeters long. Sensor S₁ isprovided at one end and sensor S₂ provided at the other end. Thediameter of the plastics tube is 110 millimeters. Each coil comprises500 turns of 32 g copper wire. The winding extends over 10 millimetersof length of the longitudinal extent of the member 10. The distancebetween the centres of each of the coils S₁ and S₂ is 600 millimeters.Fifteen 75 millimeter long bar magnets 13 are mounted centrally overeach coil and equispaced around the circumference thereof. North likepoles are arranged to be at one end of the coil. Two coaxial cablesterminate with the coil winding tails 15 and connect each of the sensorcoils S₁ and S₂ to the electronic circuitry. The supporting member 10may be arranged and held in the required position by bracket means notshown. In order to provide robustness to the coils and magnets they areencapsulated in a silicone rubber compound shown generally by thenumeral 17.

Sensor S₁ is a sensor which starts a first counter in an `up` directionand sensor S₂ is a sensor which starts a second counter which counts inthe opposite direction to that of the first counter--`down`--so thatwhen that counter reaches a known value--in this case zero--then theflash 6 can be initiated. The distance of sensor S₂ from the catcher box4 is a known distance having regard to the rates of counting of thefirst and second counters.

The velocities of the bullet 1 are in the range of 1000 to 2000 m/s.Thus, it can be seen that if the distance × is 600 milimeters and the`up` counter (triggered in response to sensor S₁ operating) runs at oneconstant rate and the `down` counter (which is triggered by S₂) runs ata faster but constant rate, that the total time of counting will vary inproportion to the bullet velocity and thus produce a trigger pulse at aconstant distance from the detectors S₁ and S₂ irrespective of thevelocity of the bullet 1. This is shown in the table below.

    ______________________________________                                                     time over                                                                              time count                                              Velocity     600 mm   down      Total Dist                                    ______________________________________                                        1000 m/s (1 mm/μS)                                                                      600 μS                                                                              250 μS 850 μS                                                                           850 mm                                  2000 m/s (2 mm/μS)                                                                      300 μS                                                                              125 μS 125 μS                                                                           850 mm                                  ______________________________________                                    

The electronic circuit shown in FIG. 3 operates as follows. The startand stop pulses from the sensors S₁, S₂ are taken to flip flop 74A and74B type DM 7474, through single stage amplifiers type MPS 6515 and thenthrough timers type LM 3905. These timers produce pulses of 250 μs eachwhich are then fed to the flip flop 74A and 74B. The outputs of thesedetermine when pulses from two oscillators 04 `up` and 04 `down` are fedto an up/down counter 192A and 192B forming part of the circuit of theflip flop 74A and 74B. While the bullet 1 is passing between the sensorsS₁, S₂ the counter 192A and 192B is counting `up` at a rate determinedby the 04 up oscillator type DM 7404. On passing the second sensor S₂only the 04 `down` oscillator type is connected and the counter thencounts down at a new rate of the 04 down counter. A zero coincidencedetector type on the output of the counter comprising the 02 and 20gates, determines when the counter has reached zero. This zero conditionis the correct point for the flash initiation and this is achieved bytriggering the output of timer type 555A, through gate 00, and thisoperates the transistor switch type MPS 3645 to fire the flash unit. The00 gate output pulse is also fed back to flip-flop 74A, 74B and thecounter 192, automatically resetting them to initial conditions for thenext shot.

A test circuit T is provided and comprises a second timer type 555Bwhich is incorporated to generate artificially simulated coil pulseswhich are applied to the circuit. The result is a fixed time indicationby the circuit and triggering of the electronic flash.

Modifications may be made to the invention as for example, instead ofthe sensors S₁, S₂ being hollow coils through which the bullet is fired,they may comprise other sensors suitable for detecting the passage of aprojectile. It is noted however, that as the two coils S₁, S₂ areidentical, the fields emanating from each will be substantiallyidentical and therefore the distance X between the coils S₁, S₂ is ameasure of the distance when the bullet disturbs the field of eachsufficient to cause a trigger pulse to be provided at a particularlevel. This pulse wil be generated by each, consequent on the bulletreaching the same magnetic strength position of the field around each ofthe respective sensors S₁, S₂. As each sensor S₁, S₂ is identical thedistance X between the coils is a measure of the distance between thosepoints on the respective fields.

In an alternative embodiment, the sensors S₁, S₂ may comprise a sheet ofplastics material coated with an electrically conductive medium wherebythe electrostatic charge, which is provided on the surface of the bullet1 consequent on its passage through the air, is detected and used tostart the operation of the counters.

These and other modifications may be made without departing from theambient of the invention the nature of which is to be determined fromthe foregoing description.

I claim:
 1. A synchronising circuit for causing a phenomena to occur ata precise point in the path of travel of an article regardless of thespeed of movement of the article in that path, comprising first andsecond spaced sensors upstream of said precise point and each operableto detect the passage of an article therepast and to provide outputsignals consequent thereon, a counting circuit which provides a count inone direction when the first sensor provides an output signal, and whichcounts in the opposite direction from the value of the count firstcounted, when the second sensor provides an output signal and circuitmeans responsive to the counting circuit returning to a predeterminedcount value to initiate the occurrence of said phenomena.
 2. Asynchronising circuit as claimed in claim 1 wherein said countingcircuit comprises two seperate counting means, one being for counting insaid one direction and the other being for counting in said oppositedirection and where said circuit means responsive to the said countingcirciut includes a zero coincidence detector connected to the outputs ofeach of said separate counting means whereby to provide an output signalusable to initiate the occurrence of said phenomena when a zerocoincidence is detected.
 3. A synchronising circuit as claimed in claim1 or claim 2 wherein said first and second spaced sensors compriseelectromagnetic sensors.
 4. A synchronising circuit as claimed in claim3 wherein said electromagnetic sensors each comprise a coil which, inuse, has a field which eminates therefrom into the path of travel ofsaid article and wherein said counting circuit is actuated by eachsensor detecting a disturbance of said field respectively, consequent onthe passage of an article therepast.
 5. A synchronising circuit asclaimed in claim 4 wherein both of said coils are mounted on a commonsupporting member to positively support and hold each apart in aposition where said article will pass each at approximatley the samedistance therefrom.
 6. A synchronising circuit as claimed in claim 2wherein the one of said two counting means which is first activated onthe passing of the article, counts at a slower rate than the other ofsaid two counting means.
 7. A synchronising circuit as claimed in claim1 wherein said article is a fired bullet and said phenomena isillumination of said bullet when it reaches a predetermined position,said circuit further comprising a lamp means for so illuminating saidbullet and a trigger circuit activatable by said circuit meansresponsive to the counting circuit.
 8. A synchronising circuit asclaimed in any one of the preceding claims assembled with a camera andan illuminating means whereby to film an article moving past said firstand second sensors when it reaches a precise point in its path oftravel.